Mattress



FIG. 1 is an isometric view of a front, top, and left side of an ornamental design for a mattress;

FIG. 2 is an elevational view of the left side of the mattress of FIG. 1, a right side being identical to the left side; and,

FIG. 3 is an elevational view of the front of the mattress of FIG. 1, a rear being identical to the front. 

The ornamental design for a mattress, as shown and described. 